Nucleoside analogues, the derivatives of the natural nucleosides found as building blocks of DNA and RNA, are effective in the clinical treatment of human cancer or viral diseases, although in the early years such compounds were evaluated as anti-tuberculosis agents. Such compounds have been registered in the market for more than 40 years, and approximately 35 products are currently in daily use. The natural nucleosides illustrated in Table 1 below, are constructed from two classes of nitrogen bases, i.e. the purines (exemplified by adenine and guanine) and the pyrimidines (exemplified by thymine, uracil, and cytosine) and from the monosaccharide ribose or deoxyribose.
TABLE 1Purines Pyrimidines Monosaccharides
The natural nucleosides all exist in the so called β-D configuration as illustrated in the Formula A below. The nitrogen base and the hydroxy-methyl side chain on the sugar ring are both on the same side (cis) of the plane of the sugar ring.

In order to obtain nucleoside derivatives with anticancer or antiviral activity, chemical modifications in either the nitrogen base and/or the monosaccharide have been performed. For instance in the nitrogen base, the addition of halogen atoms or other functional groups, insertion of additional nitrogen atoms or a stereochemical change in the monosaccharide ring from ribose to arabinose or removal of the hydroxyl group to deoxyribose may lead to products with a potential therapeutic benefit. In many products, the monosaccharide ring is conserved, while in others, the sugar ring has been changed into a chain. The nucleoside analogues are small molecules with fair to excellent aqueous solubility.
The extensive research and development effort put into the area of nucleoside analogues due to the worldwide AIDS epidemic bolstered the basic knowledge and understanding of mechanism of action, alterations in activity profile due to chemical modifications etc, are also relevant to the field of cancer treatment.
A general weakness with many drugs, including nucleoside analogues, is low activity and inferior specificity for treatment of the actual disease in question. Some of these problems may be related to the inherent activity of the drug substance itself, some may be related to certain resistance mechanisms (either inherent in the patient or acquired during treatment e.g. multiple drug resistance (MDR) in cancer treatment). Some problems may be related to certain inferior transport or cellular uptake and activation mechanisms. Some problems may be related to rapid inactivation and/or excretion of the drug.
The efficacy of nucleoside analogues depends on a large extent on their ability to mimic natural nucleosides, thus interacting with viral and/or cellular enzymes and interfering with or inhibiting critical processes in the metabolism of nucleic acids. In order to exert their antiviral or anti-cancer activity, the nucleoside analogues have to be transformed, via their mono- and di-phosphates, into their corresponding tri-phosphates through the action of viral and/or cellular kinases. As a general rule, the tri-phosphate is the active agent, but for some products, e.g. gemcitabine, even the di-phosphate may exert a clinically significant effect.
In order to reach the diseased, cancerous or virus infected cells or tissues, following either enteral or parenteral administration, the nucleoside analogues should have favorable pharmacokinetic characteristics. In addition to rapid excretion of the administered drug, many nucleoside analogues may be deactivated both in the blood stream and in tissues. For instance, cytosine derivatives, even at the mono-phosphate level, may be rapidly deaminated through the action of a class of enzymes called deaminases, to the inactive uracil analogue. The cellular uptake and thus good therapeutic efficacy of many nucleoside analogues strongly depend on membrane bound nucleoside transport proteins (called concentrative and equilibrative nucleoside transporters). Hence, compounds that do not rely on such specific uptake mechanisms are sought. Yet another activity limiting factor, particularly within the anti-cancer field, are the cellular repair mechanisms. When an anti-cancer nucleoside analogue mono-phosphate is incorporated into the cellular DNA, it should not be removed from the cancer cell DNA due to the exonuclease activity linked to the p53 protein. However, removal of a nucleoside analogue from the DNA of a healthy cell is favorable in order to limit the side effects of the drug.
Over the years, many nucleoside analogues have been developed that to a large extent overcome some or many of the activity limiting features. As an example, acyclovir (ACV) can be given to illustrate a compound with great specificity. The ACV-mono-phosphate can only be formed by viral kinases meaning that ACV cannot be activated in uninfected cells. Despite this fact, ACV is not a particularly active product. In order to circumvent the often rate limiting step in the activation of a nucleoside analogue, the intracellular formation of the nucleoside analogue mono-phosphate, several phosphonates, such as cidofovir or even mono-phosphate products, have been developed. In order to facilitate oral uptake or to secure a favorable drug disposition in the body, particular prodrugs such as Hepsera have been made.
In addition to the structural changes made to nucleoside analogues to facilitate enhanced clinical utility, further modifications have been made to improve the activity. There are several examples of modified nucleoside analogues resulting from the addition of lipid moieties (U.S. Pat. Nos. 6,153,594, 6,548,486, 6,316,425, and 6,384,019; European Patent Application Nos. EP-A-56265 and EP-A-393920; and WO 99/26958). This can be achieved by the linking of fatty acids through, for instance, an ester, amide, carbonate, or carbamate bond. More elaborate products can be made, such as phospholipid derivatives of the nucleoside analogues. See Eur J Pharm Sci 11b Suppl 2: 15-27 (2000); European Patent No. 545966; Canadian Patent No. 2468099; and U.S. Pat. Nos. 6,372,725 and 6,670,341. Such analogues are described to have antiviral activity that is particularly suitable for the therapy and prophylaxis of infections caused by DNA, RNA, or retroviruses. They are also suited for treatment of malignant tumours. The nucleoside analogue lipid derivatives may serve several purposes. They may be regarded as a prodrug that is not a substrate for deaminases, thereby protecting the nucleoside analogues from deactivation during transport in the bloodstream. The lipid derivatives may also be more efficiently transported across the cellular membrane, resulting in enhanced intracellular concentration of the nucleoside analogue. Lipid derivatives may also be more suited for use in dermal preparations, oral products (see U.S. Pat. No. 6,576,636 and WO 01/18013), or particular formulations such as liposomes (see U.S. Pat. No. 5,223,263) designed for tumor targeting.
It has been demonstrated that for nucleoside analogues with a conserved β-D configuration of the monosaccharide ring, or for nucleoside analogues with a non-cyclic side chain, the antiviral or anticancer activity can be most efficiently improved through the formation of lipid derivatives of mono-unsaturated ω-9 C18 and C20 fatty acids. See Antimicrobial Agents and Chemotherapy, Vol., 53-61 (1999); Cancer Research 59: 2944-2949 (1999); Gene Therapy, 5: 419-426 (1998); Antiviral Research, 45: 157-167 (2000); and Biochemical Pharmacology, 67: 503-511 (2004). The preferred mono-unsaturated derivatives are not only more active than the poly-unsaturated counterparts but are more crystalline and chemically stable towards oxidation of the lipid chain. Hence, they are more favorable compounds from a chemical and pharmaceutical manufacturing point of view. It has also demonstrated that the mono-unsaturated ω-9 C18 and C20 fatty acids are suited for improvement of the therapeutic activity of a large number of non-nucleoside biologically active compounds (see European Patent No. 0977725).
A relatively new subgroup of nucleoside analogues are the so called aza-C derivatives. In this class of compounds, the CH group in the 5 position in the pyrimidine base is exchanged with a nitrogen atom as shown in Formula B below.

Tumor suppressor genes that have been silenced by aberrant DNA methylation are potential targets for reactivation by these novel chemotherapeutic agents. The potent inhibitors of DNA methylation and antileukemic agents, aza-cytidine and 5-aza-2′-deoxycytidine derivatives (5-aza-C, 5-aza-CdR, Decitabine), can reactivate silent tumor suppressor genes. At high concentrations, the compounds are cytotoxic, but at lower concentrations the hypomethylation leads to differentiation of cell lines. The compounds requires metabolic activation by deoxycytidine kinase, and produces an inhibition of DNA methyltransferase. One hindrance to the curative potential of these derivatives is their rapid in vivo inactivation by cytidine deaminase (CD). The instability in aqueous solutions as well as their side effect profiles have limited clinical activity.
The present invention is directed to overcoming these and other deficiencies in the art.